Fired by dentist after insurance opened investigation

My wife had some cavities filled. She paid around $135 that day and the insurance company (in network for Ambetter) was billed for what she thought was the balance. Then she got another bill from the dentist for ~$300. She was told by the dentist that whatever the insurance didn’t pay was her responsibility and she signed an agreement about that. She said she didn’t sign the agreement and if they have it on file they should present it (they didn’t). They said they were going to write off the $300 but that was the last “handout” they would give. She called Ambetter and they said that practice by the dentist is not appropriate and opened an investigation, and she shouldn’t have paid anything up front.

Today she got a letter from the dentist saying the investigation broke trust and she needs to go somewhere else.

Question: for in network, are they generally allowed to bill beyond the negotiated insurance rate to the patient for routine services?

State: Arkansas, if that makes a difference.